1. Field of the Invention
The present invention relates to a switching power supply with regulation of the output voltage for driving variable ohmic-capacitive or ohmic-inductive loads, comprising a resonance circuit, an electromechanical energy converter, a switch, and control means.
2. Description of the Related Art
Switching power supplies with or without a resonance circuit mostly cannot do without inductive electromagnetic devices. For obtaining a low-loss switching operation, such circuits may only be operated up to a certain maximum frequency and only with resonant inductive elements or broadband transformers or inductances. Such components are volume-intensive and cause a significant cost proportion in the overall device.
For example, a self- or separate-excited half-bridge circuit is to be mentioned, which works with bipolar transistors, reverse diodes, a series resonance circuit, and inductive base feedback. An exemplary embodiment of such a half-bridge circuit is disclosed in the following document (1): S. Lowbridge, M. Maytum, K. Rutgers, “Electronic Ballasts for Fluorescent Lamps Using BUL 770/791 Transistors” (Texas Instruments, 1992). Here, the load circuit is predominantly embodied inductively, whereby low-loss switching in various load cases becomes possible. This circuit may also be classified as a class D amplifier. Even using minority-charge-free MOS (metal oxide semiconductor) transistors, it would have the disadvantage of capacitive sweep-out losses, because the switches have to be switched on under voltage unless an output-side resonance choke lets the voltage rise to about zero across the respective switch when switching on. Thus, zero voltage switching (ZVS), which distinguishes itself by a voltage across a power semiconductor being made zero before and during a switching operation when switching, is achieved by a sufficiently large (resonance) inductance at the load circuit.
Moreover, there are class E RF (radio frequency) amplifiers with only one switch and high efficiency. An embodiment of such a circuit is published in the following document (2): “N. O. Sokal, A. D. Sokal, “Class E—A New Class of High Efficiency Tuned Single-Ended Switching Power Amplifiers”, (IEEE, Journal of Solid-State Circuits, Vol. SC-10, No. 3, June 1975). Such amplifiers are largely used as transmission amplifiers and are operated with an externally-generated clock at an optimum turn-on time. The turn-on time mostly is about half a period duration (D=0.5 corresponds to optimum). Here, D designates the relative (i.e. related to a period duration) turn-on time. This circuit also needs a resonance inductance in the load circuit, but achieves the zero voltage switching (ZVS) in parallel to a sufficiently large capacity. Whereas, in a half-bridge circuit, the parallel capacity to the switch is chosen as small as possible in order to achieve the zero voltage switching (ZVS) easily by resonance inductance, this parallel capacity is made as large as possible in the class E circuit mentioned, in order to keep the maximum voltage across the switch as small as possible during switch-off. If the capacity is, however, chosen too large, the voltage can no longer return to zero, and inadmissible turn-on losses occur.
When employing high-frequency piezoelectric transformers (piezo-transformers) or other energy converters with an electromechanical energy conversion, arbitrary transformation ratios may be realized, but these devices mostly do not offer predominantly inductive input behavior. Such electromechanical converters are mostly also very narrow-band and can only transfer sinusoidal vibrations with reference to their frequency behavior. A hard-switching converter topology is therefore less suited for operation thereof. Thus, the resonance operation has to be chosen, favorably also in a resonance converter topology. Since capacitive input and output behavior is substantially given by a piezoceramic material, such a converter may only replace the conventional inductances or transformers when, in the case of a desired inductive load circuit behavior, it is seen to that there is additional inductive shaping of the load circuit. In a half-bridge circuit, such inductive load circuit behavior is demanded in order to keep the switching losses small. As the simplest measure, an additional, yet small conventional conductance may be inserted into the load circuit. If the turn-on losses are small enough due to correspondingly low input voltage levels (e.g. small voltages up to 24 V), capacitive behavior of the electromagnetic converter in the half-bridge may also be acceptable.
Finally, also switching in a resonance case using a piezoelectric transformer may be configured so that the switching losses are minimized when a re-charge rime of the relatively large input capacities of the piezoelectric transformer is bridged by exactly meeting required control times by temporarily switching off both switches (dead times). For this, however, an accurately adjustable high-side and low-side driver circuit, which further mostly comprises an integrated circuit, is required. An embodiment of such a circuit is published in the following document (3): R. L. Lin, F. C. Lee, E. M. Baker, D. Y. Chen, “Inductor-less Piezoelectric Transformer Electronic Ballast for Linear Fluorescent Lamps”, APEC2001, Anaheim, Calif., USA, Proceedings, Vol. 2, pp. 664–669.
In a class E resonance circuit according to document (2), the predominantly capacitive input behavior of a piezo-transformer is useful by the amount of the input capacity being able to be adapted to an electrically required value and thus not being spurious, as is the case in a half-bridge or another load circuit aimed at acting inductively. Such class E circuits with a piezoelectric transformer are already known from document (4), EP 0 665 600 B1.
Such circuits are, however, not employed for the case of a large input voltage and a small output voltage technically given in line voltage applications in document (4), but used for upward transformation of a smaller voltage to a larger one. This limitation to small input voltages has previously mainly been determined by the lacking availability of dynamically quick, high-blocking power switches, which can now be produced inexpensively, e.g. fieldstop IGBT (integrated gate bipolar transistor) up to 1700 V, or cool MOS transistors up to 800 V.
In small voltage applications, a class E circuit according to document (4) and according to document (2) is mostly employed in optimum operation with the relative turn-on time of D=0.5. In most cases, such a circuit requires an additional input-side parallel capacity in the case of the upward transformation, in case the input capacity of the piezoelectric transformer is not large enough. This is not given in a downward transformation case, where the input capacity of some embodiments of piezoelectric transformers may be too large.
Furthermore, there are single-transistor circuits with a piezo-transformer, which require a resonance inductance that does not, or not exclusively, act in a smoothing manner and thus has to be suited for a high frequency of about 50 to 200 kHz by a suitable choice of magnetic material and braided wire. An embodiment of such an arrangement is disclosed in document (5) U.S. Pat. No. 6,052,300. Furthermore, an input-side smoothing choke, as opposed to a smoothing or resonance inductance not acting on the input side, prevents a direct effect of high-frequency current vibrations on an input or on a smoothing capacitor, so that an input-side smoothing choke (referred to as choke inductance in the following) is to be preferred with respect to other arrangements of an inductance.
With reference to the control of circuits with a piezoelectric converter, the phase-locked loop (PLL) is a typical way of frequency tuning. In document (6), U.S. Pat. No. 5,855,968, a possibility is described to adjust the phase shift between output voltage and the driver signal of a circuit according to (4) so that a PLL circuit with a simple oscillator/driver IC can be realized. This regulating circuit for class E is particularly well suited for piezo-transformers with upward transformation properties, because the voltage maximum at the output of the transformer represents a prominent point for the nominal power at the same time. Owing to the low current stress in upward transformation, the frequency characteristic of the output voltage will almost correspond to an open case, so that the transformation ratio between open and nominal load changes little. In (6), substantially a phase-locked loop is given via the voltage courses between input and output, so that a maximum output voltage always arises when the right phase location (in this case about 90° or slightly less) is adjusted. This also applies for other topologies with strong upward transformation of the voltage, for example for the half-bridge circuit. For the case of the downward transformation, flattening of the transmission characteristic of the output voltage can be observed, since the secondary-side current stress clearly affects the voltage transmission ratio. In this case, in an inaccurate fixing of the nominal point, very different output powers arise in applications, such as power supplies, when an adjustment to the phase between the voltages would take place. When using the phase shift between the maximum of the output current and an input quantity as a basis for a regulation, the desired nominal power (nominal voltage) will hardly be adjustable by the exemplar scatterings of load (nominal current at nominal voltage) and piezo-transformer, independently of the topology. Thus, the regulation has to take place to a certain nominal value of the output current that is not necessarily the maximally transferred current. A basic solution for the adjustment of a PLL regulation according to this principle with exactly this disadvantage has become known according to (3). For the adjustment of the load current in (3), thus a very accurate regulating circuit has to be employed, which either requires a particular nominal value adjustment for each device in order to achieve the nominal point. Or the value of the output current is sampled accurately enough with great processing overhead. Phase regulation by sampling the zero crossings of output voltage and output current in a half-bridge circuit is again inaccurate owing to the scattering of re-charge times at the input of the piezo-transformer, so that evaluation of the amplitude of the output current is required there to adjust the nominal power.
When the output voltage of a current supply with changing load is to be regulated, solutions are known, which only enable little margin in the frequency position when using a conventional class E converter according to (2), as it is known from the following document (7): R. Redl, B. Molnar: Design of 1.5 MHz Regulated DC/DC Power Converter, in PCI/MOTOR-CON September 1983, Proceedings, pp. 74–87. A strong load increase particularly can no longer be reacted to by frequency change and the output voltage rises. With this, a consumer would be under too much strain. A pulsed operation of the class E amplifier may be a remedy. But with this there are disadvantages regarding voltage smoothing at the output when a supply as ripple-free as possible is demanded. Moreover, additional interferences as opposed to the line input arise, which develop by the constant upswing and turn-off of the amplifier and may require additional filtering effort. For those reasons, line current supplies with the conventional class E topology have not yet been able to gain acceptance, since they are inferior to half-bridge circuits or the most widely used flyback converter topology both in terms of regulation and with reference to the transmission behavior. In addition, they mostly require galvanic decoupling of the load circuit, which would require a further output transformer along with two chokes.
But if a piezo-transformer is inserted into the circuit according to document (6) as load circuit, the possibility arises to keep the output voltage constant over a small change in frequency. But in (6) no galvanic decoupling is achieved, although an auxiliary load circuit is employed for the generation of required phase control signals for guaranteeing a basic load. Thus, the configuration according to (6) is only suited with some reservations for current supplies with a regulated, galvanically decoupled input voltage. Since, in addition, in the case of an output rectification not shown in (6) only the AC voltage of the transformer output would be returned, additional control of the DC voltage is required. On the one hand, the flow voltage of the output rectifier may not be regarded as constant and is particularly dependent on the temperature. Furthermore, also a change of the flow voltage is to be expected at a change of load. Accordingly, a galvanic separation is required, for example via opto-coupling. If the signal is returned to the rectified output voltage in a galvanically separated manner, the additional necessity and possibility of frequency regulation via this signal results. This may, however, only take place with some reservations with the circuit shown in (6), because the phase-locked loop can only react with the average delay of half a period duration via a peak value rectification of the output voltage. With quick periodic changes in load, this also bears the risk of instability and a propensity to oscillation of the regulation. Thus, circuits according to (6) are not suited for precisely regulated DC voltage supplies with galvanic separation in spite of their simplicity. In particular, there is the danger that the output voltage breaks down, because the phase coupling is left.
According to document (10) (“A Very Simple DC/DC Converter Using Piezoelectric Transformer”, M. J. Prieto et al., IEEE, 2001, pp. 1755 to 1760), current supplies with piezo-transformers are implemented in a half-bridge topology. Despite the small constructional size of the additional resonance inductance, which is switched to the input of the piezo-transformer, this inductance causes additional effort when it has to be designed for very high frequencies. This embodiment shows all the disadvantages of the half-bridge solution with piezo-transformers for line current controls. A high-volt driver IC as well as two fast switches (transistors) are required. Substantially, the regulation may only take place via the detection of the output voltage. The input voltage section is limited by the compliance with the ZVS condition in all load cases. For this reason a great value of the input-side charge capacitor has to be chosen to obtain small devices for resonance choke and piezo-transformer, wherein the input voltage variation is kept as small as possible. For a wide-range input with e.g. 85 V to 260 V, this solution is therefore not very suitable, because a large energy magnitude has to be stored in the input-side resonance choke, so that the aim of miniaturization is hardly achieved any longer.